Technical Field
The present invention is directed toward a pump for providing a precise selected output volume and, more particularly, toward an ambulatory infusion pump insertable into a control unit having a cyclical or reciprocating drive for delivering a precise selected output volume of fluids for intravenous or intra-arterial injection into a patient.
Background Art
Use of infusion pumping devices for parenternal delivery of fluid to patients in hospitals or other patient care locations has increased dramatically in recent years. Infusion pumps have replaced gravity control infusion systems in some applications because of their greater accuracy in delivery rates and dosages and their ability to permit varying medication delivery profiles to optimize medication effectiveness. Infusion pumps have been particularly useful for delivering dangerous drugs to patients over an extended period of time in precisely measured and timed volumes to maximize drug effectiveness and patient safety.
In recent years, spiraling hospital costs and shortages of critical facilities have led to efforts to allow earlier dismissal of patients from hospitals. As a result, numerous ambulatory infusion pumps have been developed. These ambulatory infusion pumps, like the stationary infusion pumps which preceded them, require a great degree of accuracy in the administration of fluids to maximize the effectiveness of medication and to protect the patient. Typically, these ambulatory infusion pumps include a pump control unit and drive mechanism including a variety of operating controls adapted to accept a disposable pump cassette containing the pump mechanism. The control unit and pump cassette must be small and light enough to be readily carried by patients. The pump cassettes are preferably disposable to reduce the risk of spreading disease when the control units are reused and to prevent the inadvertent mixing of drugs. As a result, they must be relatively small and economically manufactured to minimize costs and encourage their use. Notwithstanding, the accuracy of their output volumes must be precisely maintained.
Ideally, infusion pumps exhibit monotonicity, a high degree of pump linearity and a high degree of variability in selection of pump granularity. Monotonicity means that an incremental physical displacement will yield an incremental volume displacement. Pump linearity is a measure of the pump's ability to apply substantially identical incremental volume displacements for each cycle. Finally, granularity is a measure of the size of each incremental volume displacement comprising a desired total volume displacement. The ideal pump exhibits these characteristics while requiring a minimum amount of power, weight and size.
In an effort to meet these requirements, the prior art has looked to different types of pumps. For example, rotary peristaltic pumps have been used, but such pumps are neither monotonic nor linear. Linear peristaltic pumps also have inadequate linearity. Syringe pumps have been complicated mechanisms which are expensive to construct and bulky. Thus, syringe pumps are not ideal for an ambulatory infusion pump.
Positive displacement pumps have also been used having an elastomeric diaphragm actuated by a solenoid controlled armature. The diaphragm acts like a piston to increase or decrease the size of a pump chamber defined by a rigid pump housing. As the volume of the pump chamber moves from its minimum to its maximum, medication is drawn into the chamber from a reservoir through a one-way input valve. As the armature drives the diaphragm into the chamber, thus decreasing the volume of the chamber, fluid is driven out of the chamber through a one-way output valve. In many of these structures, however, the diaphragm is subject to deformation due to variations in reservoir pressure or ambient pressure. As a result, the output volume is subject to variation, creating a serious problem where potentially dangerous drugs are being administered. Moreover, the valving systems of these pumps are typically spring biased, making them relatively complicated and expensive to build.
One way to overcome the variation in output volume these pumps can exhibit is shown in Fischell, U.S. Pat. No. 4,594,058. Fischell uses a structure similar to the above-described prior art, only provides a rigid housing which contorts the flexible elastomeric diaphragm in an effort to deliver a more precise, constant stroke volume independent of ambient and reservoir pressures. However, this structure is relatively bulky, complicated and expensive to build.